Back in September of 2016 when my layout was near completion but with a non working LCS, Rudy Trubitt and Jon Z built a mockup of my LCS in their lab. They confirmed the voltage was down to about 2.5V after the 8th device and 50' of cable. The final install I have with 5 of the powered joiners was as suggested by Rudy and Jon and the LCS has always worked.
The function of the Joiner is to bring the voltage back to 5V and provide additional power to supply devices downstream of its location.